Development of electrochemical sensors for the determination of selenium using gold nanoparticles modified electrodes

Sensors and Actuators B 220 (2015) 263–269

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Development of electrochemical sensors for the determination of selenium using gold nanoparticles modified electrodes Rodrigo Segura a,∗ , Jaime Pizarro a , Karina Díaz a , Alan Placencio a , Fernando Godoy a , Eduardo Pino b , Francisco Recio c,d a

Departamento de Química de los Materiales, Facultad de Química y Biología, Universidad de Santiago de Chile (USACH), Santiago 33, Chile Departamento de Ciencias del Ambiente, Facultad de Química y Biología, Universidad de Santiago de Chile (USACH), Santiago 33, Chile c Departamento de Química Inorgánica, Facultad de Química, Pontificia Universidad Católica de Chile, Av. Vicu˜ na Mackena 4860, Macul, Santiago de Chile, Chile d Centro de Nanotecnología y Materiales Avanzados, CIEN-UC, Pontificia Universidad Católica de Chile, Chile b

a r t i c l e

i n f o

Article history: Received 9 December 2014 Received in revised form 27 April 2015 Accepted 4 May 2015 Available online 3 June 2015 Keywords: Gold nanoparticles Anodic stripping voltammetry Selenium Chemically modified electrodes

a b s t r a c t Electrochemical sensors have been developed for the determination of selenium (Se (IV)) using glassy carbon electrodes modified with gold nanoparticles (AuNPs) obtained electrochemically (GC/AuNPs/E) and chemically (GC/AuNPs/C) using square wave anodic stripping voltammetry (SWASV). GC/AuNPs/E was characterized by cyclic voltammetry (CV), atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray dispersion spectrometry (EDS). The results indicate a homogeneous distribution of AuNPs with a 75 ± 20 nm particle size distribution. The chemically prepared AuNPs were characterized by transmission electron microscopy (TEM) and UV–vis spectroscopy (Surface Plasmon Band), and the results indicate that the AuNPs have a spherical shape (7.4 ± 1.3 nm). GC/AuNPs/C was prepared using a drop coating technique and was characterized by CV. The optimum conditions for the prepared electrodes were as follows: accumulation potential of −0.80 V (Eacc ), accumulation time (tacc ) of 120 s, and a frequency of 15 Hz. A linear range was observed from 10 to 50 ␮g L−1 with a limit of detection (LOD) of 0.120 ␮g L−1 for GC/AuNPs/E, and a linear range was observed from 15 to 55 ␮g L−1 with a LOD of 0.175 ␮g L−1 for GC/AuNPs/C. The proposed procedure was validated with the TM-15 certified reference material, and good accuracy and precision were observed. In addition, this approach was applied to seawater samples with satisfactory results. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Selenium is an essential element for animals, plants, and humans. However, toxic effects have been observed in specific concentration ranges, and these effects can arise from an excess or deficiency of this metalloid. The World Health Organization (WHO) established the maximum limit of Se in drinking water to be 10 ␮g L−1 [1–3]. The determination of selenium in different matrices is very important in biochemical, environmental, and chemical analyses. Selenium can be determined by high performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS) [4], hydride generator atomic absorption spectroscopy (HG-AAS) [5], graphite furnace atomic absorption

∗ Corresponding author. Tel.: +56 227181172. E-mail address: [email protected] (R. Segura). http://dx.doi.org/10.1016/j.snb.2015.05.016 0925-4005/© 2015 Elsevier B.V. All rights reserved.

spectroscopy (GF-AAS) [6], gas chromatography (GC) [7], and various electroanalytical techniques (i.e., anodic stripping voltammetry (ASV), cathodic stripping voltammetry (CSV), and adsorptive stripping voltammetry (AdSV)), which exhibit a high sensitivity and are inexpensive [8]. Electrochemical stripping techniques have been developed using mercury film electrodes (MFE) and hanging mercury drop electrodes (HMDE), which have high sensitivity and reproducibility. However, their use is restricted due to their high toxicity [9,10]. It is necessary to develop electrodes composed of environmentally friendly materials that have characteristics similar to those of the mercury electrodes. One novel alternative is the use of electrodes modified with nanomaterials, such as carbon nanotubes [11], graphene [12], and metal nanoparticles [13]. Due to their small size (1–100 nm), metal nanoparticles exhibit different physical, electronic and chemical properties compared to those of the bulk metals [14]. A solid gold electrode has been employed for the detection of arsenic [15], chromium [16], amino acids [17], and cadmium

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[18]. However, one approach of improving the sensitivity as well as the detection (LOD) and quantitation (LOQ) limits of the metals and metalloids is through the use of electrodes modified with AuNPs [19], which increase the effective surface area and mass transport to endow potential catalytic properties and excellent conductivity. These materials provide an alternative in the design and improvement of electrochemical sensors and biosensors [20]. The most common synthesis strategies for gold nanoparticles involve photochemical [21], electrochemical [22], or chemical reduction in the presence of a stabilizer (citrate or thiol) [23]. The electrodes modified with AuNPs have been used for the determination of different metals ions, such as Hg [24], Cu [25], As [26], Cr [27], Sb [28], Cd and Pb [29]. Mercury [30–33], gold [34] and silver [35] electrodes have been used with electrochemical techniques for selenium determination. However, the application of AuNPs for the determination of Se has not been previously reported. In this study, a glassy carbon electrode modified with gold nanoparticles generated by electrochemical and chemical methods has been developed as an analytical methodology for the determination of selenium. 2. Experimental 2.1. Apparatus The cyclic voltammetry and anodic stripping voltammetry experiments were carried out with a CH Instruments (USA) model 620C potentiostat. The three-electrode system consisted of a working glassy carbon electrode (CHI104, 3-mm diameter), a Ag/AgCl reference electrode (3 mol L−1 KCl), and a platinum wire auxiliary electrode set up in a 10 mL electrochemical cell. The GC/AuNPs/E surface was characterized by atomic force microscopy (AFM) (Nanoscope IIIa, Digital Instruments) in contact mode using silicon nitride (Si3N4) points. The modified electrodes were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) using a LEO 1420VP instrument with an acceleration voltage of 25 kV and a working distance of 11 mm. Transmission electron microscopy (TEM) images were recorded on a JEM 2100 HT instrument. The UV–vis absorption spectrum was recorded on an Agilent 8453 instrument using a glass cell. 2.2. Reagents All of the reagents were analytical grade. These reagents include tetrachloroauric acid (HAuCl4 99% Aldrich), trisodium citrate dihydrate (C6 H5 O7 Na3 ·2H2 O, 98% Aldrich), sodium borohydride (NaBH4 96% Aldrich), a tetrachloroauric acid trihydrate standard solution (HAuCl4 ·3H2 O, Merck), a perchloric acid solution (HClO4 , Mallinckrodt), and nitric acid solution (HNO3 , Merck) and a selenium standard solution (SeO2 in HNO3 , 1000 mg L−1 , Merck).

surface. The electrodes were rinsed and sonicated for 15 min in double-distilled water. The three-electrode electrochemical cell was washed continuously with a 0.5 mol L−1 HNO3 solution. 2.4.1. GC/AuNPs/E: In an electrochemical cell with 1.0 mL of 2.54 mmol L−1 tetrachloroauric acid trihydrate and 9.0 mL of Milli Q water, an Eacc of −0.80 V, tacc was applied for 60 s. The modified surface was characterized by SEM, AFM, and CV. 2.4.2. GC/AuNPs/C: Using a drop coating technique, 20 ␮L of a colloidal suspension of the AuNPs was added to the surface of the electrode followed by drying at room temperature. To obtain a similar electroactive surface area on both types of electrodes, several drops of colloidal suspension were added to the electrode, and the optimal volume of AuNPs was determined to be 20 ␮L. The surfaces of the modified electrodes were characterized by SEM, EDX, and AFM, and the electroactive surface areas were evaluated by cyclic voltammetry from 0.0 to 1.60 V using a scan rate of 0.1 V s−1 in 1.0 mol L−1 HClO4 . A final volume of 10 mL was achieved by adding MilliQ water (18 M cm). 3. Results and discussion 3.1. Characterization of GC/AuNPs/E The GC/AuNPs/E was characterized using AFM and SEM analyses. The AFM topographic image of GC/AuNPs/E (Fig. 1a) shows the homogeneous distribution of AuNPs, which had a cross section (Fig. 1b) with an average size of 75 nm and a roughness of 24.7 nm. The SEM analysis indicated a homogeneous distribution of AuNPs with an average size of 75.82 ± 20 nm, which is similar to the results obtained using AFM and EDX confirming the presence of gold on the electrode surface (Fig. 1c and d). 3.2. Characterization of GC/AuNPs/C The chemically prepared gold nanoparticles were characterized by TEM and the surface Plasmon band (SPB). Fig. 2a shows the TEM image of the chemically prepared AuNPs, and the results indicate that spherical nanoparticles with an average diameter of 7.4 ± 1.3 nm were formed. This result is in good agreement with the SPB, which had a maximum absorption wavelength at 517 nm (Fig. 2b) and is in agreement with the literature results for spherical nanoparticles with a diameter of less than 9 nm [37,38]. Due to the small size of the AuNPs and their good distribution on the electrode, no relevant images were obtained by SEM and EDX. 3.3. Electrochemical characterization of GC/AuNPs/E and GC/AuNPs/C

2.3. Preparation of AuNPs The chemical synthesis was performed according to a previously reported protocol [36]. 0.1 mol L−1 NaBH4 was added with stirring to an aqueous solution that contained 20 mL of 0.25 mmol L−1 HAuCl4 and 0.25 mmol L−1 trisodium citrate. The AuNPs were characterized by TEM and UV–vis spectroscopy. 2.4. Preparation of electrodes The glassy carbon electrodes were mechanically polished using 0.3 ␮m alumina with double-distilled water on a porous

Fig. 3 shows the voltammograms of the gold (Au), GC/AuNPs/E and GC/AuNPs/C electrodes at 0.1 V s−1 . The modified electrodes with AuNPs contain two anodic peaks at 0.93 and 1.12 V while the Au electrode at 1.24 and 1.46 V, which are associated with the formation of gold oxides. The cathodic peak was shifted to more negative potentials with the decrease in size of nanoparticles from 0.80 V (Au) to 0.70 V (GC/AuNP/E) and 0.68 V GC/AuNPs/C corresponded to the reduction of the formed gold oxides [39–42]. On the other hand, the oxygen evolution starts at less positive potentials to the electrodes containing AuNPs than gold electrode and the residual current was higher in the electrodes with

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Fig. 1. AFM image of GC/AuNPs/E: (a) topography; (b) cross section, (c) SEM image, and (d) EDS analysis of the surface of GC/AuNPs/E.

nanoparticles, these changes may be due to the amount of exposed carbon and gold as well as aggregation on the electrode surface [43]. According to a previous study, the charge required to reduce a monolayer of gold oxides on a polycrystalline gold surface in acidic medium is 489 ␮C cm−2 [44]. Based on the integration of

the cathodic peak at 0.70 V, a charge (Q) of 27.2 ␮C, which corresponds to an electroactive area of 0.056 cm2 , was determined for GC/AuNPs/E. For GC/AuNPs/C, a charge of 27.8 ␮C with an electroactive area of approximately 0.057 cm2 was determined. Therefore, no differences between the electroactive areas under our experimental conditions were observed.

Fig. 2. (a) TEM image of the chemically synthesized AuNPs and (b) UV–vis spectrum of the colloidal suspension of AuNPs.

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signal at 0.80 V (0.20 ␮A) for (Au), 0.70 V (3.00 ␮A) for GC/AuNPs/E and 0.74 V (3.20 ␮A) for GC/AuNPs/C due to Se (IV). The shift towards more negative potentials for the oxidation of Se (IV) relative to the gold electrode was due to the catalytic effect of the AuNPs and aggregation on the electrode surface, generating an increase in the current response. 3.5. Effect of experimental variables

Fig. 3. Cyclic voltammetry of the Au, GC/AuNPs/E, and GC/AuNPs/C electrodes in 1.0 mol L−1 HClO4 . Scan rate = 0.1 V s−1 .

3.4. Electrochemical performance of selenium In acidic media, the reduction of the electroactive Se (IV) species has been proposed to occur via different mechanisms based on the applied potential. When the reduction occurs at positive potentials, the reaction shown in equation (1) may occur preferentially [45]. H2 SeO3 + 4H+ + 4e− → Se + 3H2 O

(1)

Some authors have proposed that at negative potentials, the reduction reaction of Se (IV) to Se (−II) occurs according to equation (2) [46,47]. H2 SeO3 + 6H+ + 6e− → H2 Se + 3H2 O

(2)

However, H2 Se can rapidly react with selenious acid to yield elemental selenium according to equation (3): 2H2 Se + H2 SeO3 → 3Se + 3H2 O

(3)

Because the reduction of Se (IV) may occur by different mechanisms, the net process that occurs on the surface of the electrode is considered to involve the reduction of Se (IV) to Se (0). For the Au, GC/AuNPs/E and GC/AuNPs/C electrodes, Fig. 4 shows the SWASV voltammograms of the supporting electrolyte (i.e., 1.0 mol L−1 HClO4 ), which does not exhibit electrochemical signals. A 50 ␮g L−1 Se (IV) standard was added to generate an oxidation

3.5.1. Accumulation potential (Eacc ) The effect of the Eacc on the current response of 50 ␮g L−1 Se (IV) on GC/AuNPs/E and GC/AuNPs/C between 0.20 and −1.20 V is shown in Fig. 5a. All measurements were done in triplicate (n = 3) with % RSD less than 5%. For both electrodes, the current increased as the potential became more negative than −0.20 V due to initiation of the reduction of Se (IV). At potentials lower than −0.70 V, a current plateau was observed for GC/AuNPs/E, which is associated with the complete coverage of the surface. For GC/AuNPs/C, the current increased up to −1.20 V due to the formation of multiple layers of AuNPs [48]. At potentials lower than −1.20 V, the reduction of the proton to hydrogen (H2 ) begins at both electrodes. To compare the response of both electrodes to selenium, −0.80 V was chosen as the optimum accumulation potential. 3.5.2. Accumulation time (tacc ) In this study, a concentration of 20 ␮g L−1 Se (IV) was used to avoid the rapid saturation of the active sites on the electrodes. Fig. 5b shows the dependence between the current and accumulation time using the two prepared electrodes. For GC/AuNPs/E, an increase in the current was observed between 60 and 180 s, and a maximum current was observed at 200 s. Then, the current reaches a constant value due to the saturation of the electrode surface. However, the saturation time for GC/AuNPs/C occurred at 660 s, which may be due to the smaller size of the nanostructures generated by chemical synthesis resulting in saturation of the system. Therefore, higher analyte concentrations or longer accumulation times were required. An accumulation time of 120 s was determined to be optimum for the analytical validation. 3.5.3. Frequency The effect of the SWASV frequency on the current signal was studied for both electrodes. The results in Fig. 5c indicate an increase in the current with frequency with a maximum at 95 and 70 Hz for GC/AuNPs/E and GC/AuNPs/C, respectively. However, the peak current is wider with less definition and increasing residual currents. To obtain defined signals and avoid high residual currents, a frequency of 15 Hz was chosen for both electrodes. 3.5.4. Interference The possible interferences of Sb (III), Cu (II), Fe (III), Pb (II) and As (III) on the anodic stripping voltammetry of Se (IV) was investigated by addition of an interfering ion at 20 and 100 ␮g L−1 to a solution containing 30 ␮g L−1 Se (IV) under the optimized conditions. The peak current of the Se (IV) was not affected. The only peak that appeared in the studied potential range was copper at 0.048 V, which did not interfere with the Se (IV) signal. 3.6. Calibration data and application to real samples

Fig. 4. Electrochemical performance of Se (IV) using SWASV for the Au, GC/AuNPs/E, and GC/AuNPs/C electrodes in 1.0 mol L−1 HClO4 and 50 ␮g L−1 Se (IV).

The analytical method was assessed under the following conditions: Eacc : −0.80 V, tacc : 120 s, and frequency: 15 Hz. Fig. 5 shows the voltammograms (a) and calibration curve (b) obtained under the optimum parameters for GC/AuNPs/E. The linear range is 10–50 ␮g L−1 with a LOD and LOQ of 0.120 and 0.402 ␮g L−1 , respectively, and a Pearson coefficient of 0.997.

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Fig. 5. (a) Effect of the accumulation potential using 50 ␮g L−1 Se (IV) and tacc = 120 s. (b) Effect of accumulation time using 20 ␮g L−1 Se (IV) and Eacc = −0.80 V. (c) Effect of frequency using 50 ␮g L−1 Se (IV), Eacc = −0.80 V and tacc = 120 s for GC/AuNPs/E () and GC/AuNPs/C () in 1 mol L−1 HClO4 .

Fig. 6. Square wave stripping voltammograms and calibration curves for Se (IV) using GC/AuNPs/E (a–b) and GC/AuNPs/C (c–d) at 1.0 mol L−1 HClO4 , Eacc = −0.80 V, tacc = 120 s, and f = 15 Hz.

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Table 1 Results for the determination of Pb (II) in seawater sample. Sample

Seawater

Added (␮g L−1 )

20

Found (␮g L−1 )

Electrode

GC/AuNPs/E GC/AuNPs/C

% Recovery

ICP-EOS

SWASV

20.65 ± 0.40

19.44 ± 0.90 20.61 ± 0.82

94.14 99.81

Table 2 Selected published data for the application different electrodes in the stripping voltammetry of Se (IV). Electrode type

Mode

Linear range (␮g L−1 )

Detection limit (␮g L−1 )

HMDE BIFE

DPCSV DPAdSV

1.2–75 2.0–30

0.1

Au modified with Poly 3, 3 -diaminobenzidine 4HCl-Nafion Screen printed graphite electrode BDD modified with AuNPs Poly (3, 3 -diaminobenzidine) film on a gold electrode Renewable silver annular band working electrode GC/AuNPs/E GC/AuNPs/E

DPASV DPASV DPASV DPASV DPCSV SWASV SWASV

0.4–158 10–1000 10–100 7.9–79 1.0–10 10–50 15–55

Fig. 5(c) and (d) shows the voltammograms and calibration curve for GC/AuNPs/C obtained in a linear range of 15–55 ␮g L−1 with LOD and LOQ of 0.175 and 0.582 ␮g L−1 , respectively, and a linear regression equations with correlation coefficients greater than 0.999 (Fig. 6). To evaluate the accuracy of the developed method, the electrodes were tested using a certified reference material (TM-15) with a concentration of 14.5 ␮g L−1 Se (IV). The analyses were carried out in triplicate, and relative errors of 6.92% and 3.08% were obtained for GC/AuNPs/E and GC/AuNPs/C, respectively, with no significant differences between the theoretical and experimental values. To test the repeatability, GC/AuNPs/E and GC/AuNPs/C were prepared in quadruplicate, and the concentration dispersion of a synthetic sample with 10 ␮g L−1 Se (IV) was evaluated, which resulted in RSDs of 17.7 and 16.6% for (GC/AuNPs/E) and (GC/AuNPs/C), respectively. The proposed method was applied to the determination of Se (IV) in a seawater sample obtained from Quintero Beach, Chile, which was filtered through a 0.45 ␮m membrane. The sample was analyzed by inductively coupled plasma spectrometry (ICP-OES), and the presence of selenium was not detected. However, this sample was fortified with 20 ␮g L−1 Se (IV) and analyzed by ICP-OES and ASV. The results obtained using both methods were compared (Table 1) and indicated that there are no significant differences with a recovery near 100%. The results obtained with stripping voltammetry are reported in Table 2.

4. Conclusions The proposed method was successfully employed for the detection of Se (IV) using square wave anodic stripping voltammetry with GC/AuNPs/E and GC/AuNPs/C electrodes in seawater samples with good accuracy and precision. Acceptable agreement was obtained between our results and those from a certified reference material.

Acknowledgments We wish to thank “Fondo de Desarrollo Científico y Tecnológico” (FONDECYT), Chile for financial support under project N◦ 1140206 and “Dirección de Investigación Científica y Tecnológica” (DICYT), Universidad de Santiago de Chile.

0.06 4.9 0.78 0.15 0.12 0.17

Matrix

Reference

Milk Multi-vitamin tablets Human hair Seawater Tap water Standards Water quality control Surface waters Seawater Seawater

[30] [49] [50] [51] [52] [53] [54] This work This work

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Biographies Rodrigo Segura is Assistant Professor, Faculty of Chemistry and Biology, University of Santiago de Chile of Chile in Analytical Chemistry. He received his Ph.D. in 2005 in Catholic University of Chile. His research interest comprises the optimization of electroanalytical procedures employing chemical modified electrode by the determination of trace metals in different matrices. Jaime Pizarro Received his BS degree in Chemistry, in Chemistry and Biology Faculty, Santiago University (2014). His research interests cover synthesis of nanomaterials and their applications in sensors. Karina Diaz is an undergraduate student of Chemistry at the Chemistry and Biology Faculty, Santiago University. Alan Placencio is an undergraduate student of Chemistry at the Chemistry and Biology Faculty, Santiago University. Fernando Godoy is Associate Professor, Faculty of Chemistry and Biology, University of Santiago de Chile. Ph.D., Pontificia Universidad Catolica de Valparaiso, Chile (2003) in Inorganic Chemistry. He has contributed in the area of synthesis and characterization of organometallic complexes containing cyclopentadienyl and tetramethylcyclopentadienyl functionalized ligands (phosphine, amino, sulphur, crown ether and mixed-aza-macrocyles). These kinds of complexes can be used as catalyst and as potential sensor for the quantification of metal cations. Eduardo Pino is currently working in physical chemistry at University of Santiago of Chile. He obtained his Ph.D. in 2004 in the University of Santiago of Chile. My research lines involved two principal focus, the study of photophysical and photochemical of organic molecules in homogeneous and heterogeneous system. And the second in the heterogeneous photocatalysis with catalytic nanomaterials modified with gold nanoparticles. Francisco Recio postdoctoral research fellow at Santiago University of Chile. His current research interests include nano-electrochemistry and electrocatalysis.